The present invention relates to a process for polymerising vinyl chloride in aqueous suspension to give polymers possessing improved initial heat stability. It relates more particularly to a process in which polymerisation is carried out in the presence of dialkyl peroxydicarbonate initiators.
Dialkyl peroxydicarbonates are known to be particularly active initiators. They therefore possess the advantage of improving the productivity of polymerisation plants. Unfortunately the resulting polymers lead to shaped articles such as tubes, tubular film and bottles whose initial heat stability leaves much to be desired.
The initial heat stability is the ability of the polymer to withstand the degradation caused by the increase in temperature to which it must be subjected for incorporating various additives and for processing it. Poor initial heat stability manifests itself by a deterioration in the initial colour which is more pronounced the lower the stability.
It has already been suggested to improve stability by a treatment subsequent to polymerisation which consists in contacting the polymers with an aqueous alkaline solution, preferably containing a phenolic antioxidant (Canadian Pat. No. 865,802 of Feb. 9, 1970 to Esso Research and Engineering). Vinly chloride polymers which have undergone such a treatment exhibit slightly improved heat stability in certain specific formulations such as those containing tin. However, the initial heat stability is generally inferior to that of untreated polymers as far as many formulations are concerned. The effectiveness and applications of this treatment are therefore very limited.